Our previous studies indicated that the polyamine spermidine plays an important regulatory role in the development of mammary epithelium. To elucidate the control mechanism for spermidine biosynthesis, we have purified S-adenosyl-L-methionine decarboxylase, a rate-limiting enzyme, to apparent homogeneity by various chromatographic procedures, and studied various properties of the enzyme. The enzyme activity is markedly stimulated by putrescine (Ka equals 0.5 micron) which lowers the apparent Km for the substrate and also prevents inactivation of the enzyme. In contrast, spermine inhibits the enzyme activity by reducing Vmax of the enzyme. Since putrescine and spermine are intimately related to the decarboxylase reaction in the polyamine biosynthetic pathway, they may act as regulatory factors in cellular control of polyamine biosynthesis via their action on the decarboxylase. To pursue further the elucidation of the mechanism of action of spermidine which mimicks the action of glucocorticoid on milk-protein synthesis in cultured mammary cells, we have examined the effect of glucocorticoid on the transcription of casein mRNA. The results show that the steroid, in the presence of insulin, enhances the accumulation of the mRNA, which remains untranslated in the absence of prolactin. BIBLIOGRAPHIC REFERENCE: Kano, K. and Oka, T.: Polyamine transport and metabolism in mouse mammary gland. General properties and hormonal regulation. J. Biol. Chem. 251: 2795-2800, 1976.